Double patterning is a technology developed for lithography to enhance the feature density. Typically, for forming features of integrated circuits on wafers, lithography technology is used which involves applying a photo resist and defining patterns on the photo resist. The patterns in the patterned photo resist are first defined in a lithography mask, and are implemented either by the transparent portions or by the opaque portions in the lithography mask. The patterns in the photo resist are then transferred to the manufactured features.
With the increasing down-scaling of integrated circuits, the optical proximity effect posts an increasingly greater problem. When two separate features are too close to each other, the space and/or pitch between the features could be beyond the resolution limit of the light source. To solve such a problem, double patterning technology is one solution. The closely located features are separated into two masks of a same double-patterning mask set, with both masks used to pattern the layer. In each of the double-patterning masks, the distances between features are increased over the distances between features in a single mask, and hence, the resolution limit can be overcome.